The invention concerns in general a photographic imaging device and in particular a still camera and an electronic flash unit.
Electronic flash units having three power stages are known. They are capable of being combined with a still camera to produce a photographic imaging device. The variations in power stages makes it possible to adjust the luminous power of the flash unit rather accurately to accommodate variations in the focal distance and/or the diaphragm setting desired. Depending upon which power stage is activated, the flash unit has a different guide number presuming the sensitivity of the film remains constant. The guide number is equal to the product of the diaphragm aperture number, and the distance and thus a single guide number will have many different combinations of aperture and distance settings. These combinations may be taken from one or several tables and set manually on the flash unit (power stage switch) and on the camera (distance and aperture settings).
This manual setting of the flash power level is laborious and time consuming for the user and does not permit a rapid sequence of pictures to be taken of objects located at different distances. Additionally, the tables necessary for the determination of the exact amounts of light require considerable mental effort by the user and can lead to multiple errors which results in incorrectly exposed photographic images.
Electronic flash units are also known which can be combined with a camera and are equipped with an automatic exposure meter and a flash limiting device. In these so-called "computer flash units," the light emitted by the flash tube and reflected by the object of the camera is measured. The emission of the flash is automatically interrupted following the emission of a predetermined value required for the accurate exposure of the photographic image. Such computer flash units require circuitry of considerable complexity, which is reflected in the relatively high price of such instruments. Furthermore, such computer flash units operate within a rigidly predetermined range of distances. At a given film sensitivity, a certain "working aperture" must therefore be set. If the film sensitivity is changed and the distance range remains constant, the so-called "working aperture" must be adjusted to the different film sensitivity.
With these computer flash units there is again a so-called "aperture number calculator" provided in the form of a table, whereby the aperture setting corresponding to the given sensitivity of the film may be determined. The table also indicates the maximum distance permissible for the aperture setting determined. The aperture number must be set on the camera, together with the distance desired, which must be set on the camera, which must be less than the maximum distance given in the table. With the computer flash units of this type it is therefore again necessary to laboriously determine the values to be set on the camera when using the flash unit. This is inconvenient and time consuming and because of the mental involvement of the user and the transfer of values to the camera, the process can generate a multitude of sources of error, ultimately leading to incorrectly exposed images.